Mobile communications traffic is distributed over frequency bands used by several radio systems, such as different GSM systems (Global System for Mobile telecommunications). Therefore such models that operate in at least two radio systems are common among the mobile stations. The multi-band ability means of course that the design of the mobile terminals antenna will be more difficult. The design process becomes still more difficult if the antenna must be placed within the cover of the device to provide convenient use.
An antenna, which is located within a small-sized radio device and which has sufficiently good radiation and receiving characteristics, is most easily realised as a planar structure: the antenna comprises a radiating plane and a ground plane, which is parallel to the radiating plane. In order to facilitate the matching the radiating plane and the ground plane are usually interconnected at a suitable point by a shorting conductor, which creates a structure of the PIFA type (Planar Inverted F-Antenna). In principle it is possible to increase the number of operating bands by dividing the radiating plane with the aid of non-conducting slots into branches, which seen from the shorting point, have different lengths, so that the resonance frequencies of the antenna parts corresponding to the branches will be located at the desired frequency bands. However, then it is problematic to obtain the antenna matching and to get a sufficient bandwidth, at least at some of the bands. In a planar antenna a new operating band can be obtained also by using a slot radiator. Also in this case a non-conducting slot is made in the radiating planar element. The end of the slot, which opens to the edge of the planar element, is relatively close to the feeding point of the antenna. If further the length of the slot is suitable, then an oscillation is excited at the desired frequency. In the case of a two-band antenna the slot resonates for instance at the upper operating band and the conducting plane at the lower operating band.
The provision of a sufficient bandwidth or bandwidths may be problematic also using a slot radiator. One solution is to increase the number of the antenna elements: an electromagnetically connected, i.e. parasitic planar element is located close to the radiating plane proper. Its resonance frequency is arranged to be close to the resonance frequency for instance of the slot radiator, so that there is formed a uniform and relatively wide operating band. Disadvantages in using parasitic elements are that they require space, increase the production costs for the antenna and reduce the reproducibility in the production. In a corresponding manner the resonance frequency of a slot radiator and the upper resonance frequency of the two-band PIFA can be arranged close to each other, so that there is formed an uniform, relatively wide operating band. In that case the radiating plane has two slots: one slot in order to form a two-band PIFA and a second slot to form the slot radiator.
From the application publication FI20012045 there is known a planar antenna structure shown in FIG. 1. It has a ground plane 110 and a rectangular radiating planar element 120 supported above the ground plane by a dielectric frame 170. The antenna's feeding point F and the shorting point S are located at the edge of the planar element 120, on one long side. The first slot 131 of the planar element starts at the same edge, on the farther side of the feeding point, as seen from the shorting point. This first slot is arranged to operate as a radiator in the manner described above. The most substantial feature of the antenna is that now the planar element 120 in addition has a second slot 132 that starts from the edge of the plane element between the feeding and shorting points and ends at the inner region of the plane. The antenna is a dual-band antenna, and it has three resonances, which are substantial regarding its operation: the planar element 120 has a conductor branch B1, which starts from the shorting point S and extends around the end of the first slot 131, and which together with the ground plane forms a quarter-wave resonator, operating as a radiator on the lower operating band of the antenna. The first slot together with the surrounding conductor plane and the ground plane resonates and operates as a radiator on the upper operating band of the antenna. The second slot 132 is also dimensioned so that it together with the surrounding conductor plane and the ground plane forms a quarter-wave resonator operating as a radiator on the upper operating band of the antenna. The resonance frequencies of the two slot radiators can be chosen so that the upper operating band will be very wide. It extends well over the frequency bands of for instance the GSM1800 and GSM1900 systems. At the edge of the planar element, on the short side closest to the shorting point S there is extension 125 being directed towards the ground plane, which extension improves the matching of the second slot radiator and also the plane radiator.
In the structure according to FIG. 1 the exceptionally wide upper band is obtained particularly with the aid of the slot extending between the feeding and shorting points. A disadvantage of this structure is that said arrangement impairs the matching of the antenna on the lower operating band, particularly when the aim is an antenna with a minimal size.